Like the deep note but it's a bit loud even though it's the quietest version available of the manta exhaust.
Will it quieten down much as it carbons up or should I fit the standard bimodal flaps to the manta mufflers?
Thanks in advance for your advice.

I doubt it will quieten down much over time.
with the bimodal flaps you need to workout whether your mufflers are like this design

you see the valve is on the "straight through" pipe so when it is open you are essentially deleting the muffler and when it is closed the muffler will run the gasses through the baffles and quieten it down.
if your muffler is already running everything through the baffles then adding the bimodal valves wont change too much apart from turning the dual outlet into a single so it might drop the noise a bit but not as much as you need.

if you have the muffler like the picture with a straight through type pipe then adding the bimodal valve to that outlet will quieten it down a fair bit.

Its the cats helping make it louder.
If they are 100 cell cats you may as well have no cats,they are there for looks mostly,maximum flow and noise to go with it.
The noise difference between the same cat back system with stock cats and 100 cell cats is huge.

If it's any help, I've just had a new exhaust system installed and as others have said, will get louder if anything. Now have 1000ks on it.

No blower or cam on mine, but with engine-back 3" with 1 7/8 headers (all DPE) bimodals made a decent enough difference.

Not sure if its the same as the pictured muffler design above, but for me makes enough of a difference to not worry the neighbours, which is really the only reason I had them fitted - that and being able to relax at stop lights with a copper behind you.

If it's any help, I've just had a new exhaust system installed and as others have said, will get louder if anything. Now have 1000ks on it.

No blower or cam on mine, but with engine-back 3" with 1 7/8 headers (all DPE) bimodals made a decent enough difference.

Not sure if its the same as the pictured muffler design above, but for me makes enough of a difference to not worry the neighbours, which is really the only reason I had them fitted - that and being able to relax at stop lights with a copper behind you.

Big enough difference at idle with this setup to make it worthwhile.

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Ive got a Vf ssv ls3 S2. No cam & no supercharger. Just a custom tune & otr.
Ive thought about doing the same thing but 2.5 inch instead from engine back. 1 7/8 headers all Harrop/HM with bimodals.
How much of a difference do the bimodals make in terms of noise reduction at idle and full throttle? Do the bimodals quieten the car down heaps?
I want it to be reasonably quiet but roar when i put my foot down.
Maybe 200 cell cats?
Dont want to piss off the neighbours, the mrs or the coppers.

Ive got a Vf ssv ls3 S2. No cam & no supercharger. Just a custom tune & otr.
Ive thought about doing the same thing but 2.5 inch instead from engine back. 1 7/8 headers all Harrop/HM with bimodals.
How much of a difference do the bimodals make in terms of noise reduction at idle and full throttle? Do the bimodals quieten the car down heaps?
I want it to be reasonably quiet but roar when i put my foot down.
Maybe 200 cell cats?
Dont want to piss off the neighbours, the mrs or the coppers.

Click to expand...

Mate of mine has pretty much same setup as strattos on his director and he said exactly the same thing. Bimodal makes a good difference and worthwhile enough to install it to keep the noise down when required.
I'll be doing the same. DPE big boy system has 3" 200 cell cats anyway. 3" mid section but tapers to 2.5" after the big center hotdogs.